After reading through the comments I would like to know the following: Why are there no priorities?

I can't figure it out from the answers. I think with root privileges it should just be possible to say "GUI has higher priority" etc... Then when there is a memory issue you kill some low-priority processes to get the memory back.

But whatever any sane person considers part of the operating system because it is the bare-minimum of what is required to do stuff (filesystem, gui, ...) needs to have priority and always be fast. This can be defined by the distribution using the startup privileges.

So, why is this so difficult?